The invention relates to a high-pressure sodium discharge lamp intended to be operated in conjunction with an internal starter without a glow discharge or with an external starter, this lamp having an outer bulb or envelope which is provided with a lamp cap and encloses an evacuated space in which a discharge vessel is arranged.
A lamp of the aforementioned kind is known, for example, from European Patent Application No. 0 132 88 to which U.S. Pat. No. 4,620,131 corresponds. This kind of lamp is frequently used inter alia in public illumination because of its high luminous efficacy, its comparatively long life and the agreeable colour of the emitted visible radiation.
The discharge vessel of the lamp is generally made of a ceramic metal oxide, such as, for example, polycrystalline densely sintered aluminium oxide or monocrystalline sapphire. The discharge vessel has as a filling constituent sodium amalgam, which is generally dosed in excess quantity. In the lamps with excess quantity dosed the vapour pressure is determined in the operating condition of the lamp by the temperature of the excess of condensate, generally known as the coldest spot temperature.
High-pressure sodium discharge lamps have a high ignition voltage, as a result of which the lamps are operated in practice in conjunction with a starter. The lamp according to the invention is intended to be operated in conjunction with an internal starter without a glow discharge or with an external starter. The term "external starter" is to be understood in this description to mean a starter which is spatially separated from the evacuated space enclosed by the outer bulb, in contrast with the lamps in which a starter is arranged in the evacuated space enclosed by the outer bulb. The external starter may then in principle be arranged in an arbitrary position with respect to the lamp, for example in a luminaire in which the lamp is mounted or in a post to which the luminaire is secured. A further possibility consists in that the starter is arranged in the lamp cap.
It has been found in practice that in the known lamp the lamp voltage exhibits in the course of the life of the lamp after mostly an initial decrease a continuous increase. For lamps operated with alternating voltage, this also leads to an increase of the reignition voltage after each change of polarity of the supply voltage. The continuous increase of the lamp voltage and of the reignition voltage results in the lamp being extinguished and hence the end of the life of the lamp being reached. In general, a decrease of lamp voltage leads to a decrease of power and to a decrease of the lumious flux of the lamp.